The invention concerns a process for the manufacture of coarse aluminum hydroxide, for example via the Bayer process, in which bauxite is digested with an aqueous sodium aluminate liquor, so that the aluminum oxide contained in the bauxite goes into solution. The crystallization (hereinafter referred to as decomposition) of aluminum hydroxide results from the seeding of the filtered, supersaturated sodium aluminate solution with finely divided aluminum hydroxide.
In particular, it concerns a process for the crystallization of an aluminum hydroxide of coarse particle size, which contains a maximum of approximately 15 percent by weight of the particles of a particle size under 45 micrometers, from a supersaturated, alkaline aluminate solution obtained for example from the familiar Bayer process.
The decomposition proceeds in two stages through the addition of aluminum hydroxide seed of different constitution in the referred to aluminate solution in each of the two stages.
Two processes for the manufacture of aluminum oxide by the Bayer process find large scale application today, namely that usual in European plants and that practiced in American plants.
The process usual in European plants employs a high Na.sub.2 O concentration, up to 140 g/l, for the decomposition. So that a good productivity may be obtained at this high liquor concentration, the decomposition is carried out using a sufficiently great amount of seed hydroxide, for example 200-250 g Al(OH).sub.3 /l (and more) at a sufficiently low enough temperature, for example at 55.degree. C. or less. Productivities of up to 80 g of aluminum hydroxide per liter are thereby obtained. However, the precipitated hydroxide in such plant is finer than that produced in American plants. As long as the fine hydroxide from these plants is calcined at high temperatures, an oxide with little tendency towards dustiness is obtained. However, the introduction of the dry scrubbing of waste gases demands an oxide with a BET-surface area between 30 and 60 square meters per gram, which can only be obtained by a weak calcination of the aluminum hydroxide. However, a weak calcination of the fine aluminum hydroxide produced in European plants gives an oxide with a strong tendency to dustiness, which is not readily acceptable to the consumer.
The process practiced in American plants is so designed to produce a coarse hydroxide, which, under the weak calcination usual in these plants, results in an oxide with little tendency to dustiness. In order to manufacture a coarse hydrate, a liquor concentration normally under 110 grams Na.sub.2 O caustic per liter is chosen in the American process. The starting temperature is high, for example 70.degree. C., and the amount of seed material low, for example 50-120 grams Al(OH).sub.3 per liter. If too low a starting temperature for the decomposition is chosen, and the amount of seed hydroxide too high, then a fine product is obtained. The conditions in the American process for the production of the desired coarse product are in opposition to a good liquor productivity. The lower liquor productivity of this process is shown in that at best about 55 grams of aluminum oxide per liter of liquor is produced in opposition to a productivity of up to 80 grams per liter from the European process. Expressed in another way, under the American process, 18-20 cubic meters of liquor need to be decomposed to produce a ton of aluminum oxide, compared to only about 13 cubic meters for the European process.
As already referred to above, a coarse aluminum hydroxide, as produced in American plants and not produced in European plants, is required for the production of a weakly calcined aluminum oxide with a BET surface area of 30 to 60 square meters per gram. European plants could very well adapt the American practice, however, the productivity of the European plants would fall about 30-40 percent, with a corresponding rise in heat consumption per ton of aluminum oxide. It is therefore very desirable that the European plants have a process which allows the production of a coarse aluminum hydroxide, without however having to accept a reduction in capacity of the plant with a corresponding increase in the specific heat consumption of the manufactured product.
On the other hand, it is very desirable to lift the productivity of American plants to the level of European plants while maintaining the coarse product quality. Such an improvement in the American plants would mean an increase in capacity, accompanied by a reduction in the specific heat consumption per ton of manufactured aluminum oxide.
To the present time, there has been no lack of proposals as to how this aim of a coarse product and high productivity may be achieved. In U.S. Pat. No. 2,657,978, it is suggested to modify the American process so that the seed material addition proceeds in two steps. In the first step, only sufficient seed is added to promote a strong coarsening. This is then followed by a second addition so that a good productivity is achieved. By the fixing of the referred to caustic concentration at about 85 grams per liter Na.sub.2 O, a productivity of about 48 grams Al.sub.2 O.sub.3 per liter of liquor to be decomposed may be arrived at for this process. This may be compared with a productivity of about 45 grams per liter for the unmodified process with a single addition of seed, where a reaction time of 35 hours is employed in both cases. The increase in productivity is thus calculated to be about 6.5 percent.
In FR Pat. No. 1,391,596 a two-stage process with two decomposing chains is described, which, with a decomposition time of 30-40 hours, results in a productivity increase of 6.4% and a coarser product than that obtained with the normal one-stage American process. Although no absolute figures as to the productivity are given in this patent, this would not greatly exceed that in the previously quoted U.S. Pat. No. 2,657,978. The process consists of two decomposition chains, wherein one recieves fine seed in an amount and under such conditions which allows agglomeration to occur, and where the other is treated with coarse hydroxide in an amount and under such conditions that growth of the crystals results. Following the separation of the coarse product hydroxide and the coarse seed, the partially exhausted aluminate liquor from both chains is treated with further fine seed in the second stage, in order to exhaust the aluminate liquor still further, and to raise the productivity of the precipitated aluminum hydroxide. The main feature of this process is a coarse, abrasion resistant product obtained at an improved productivity.
In U.S. Pat. No. 3,486,850, a process is projected, where the increase in productivity of the American process is obtained by intermediate cooling during the decomposition, while maintaining the production of a coarse product. However, this must be carried out in a narrowly defined temperature region, in order not to obtain too fine a product. In one example with this process, a productivity of 51 grams of Al.sub.2 O.sub.3 per liter of liquor to be decomposed is quoted, with a decomposition time of approximately 40 hours.
In Light Metals 1978, Volume 2 (Proceedings of sessions 107th AIME Annual Meeting, Denver, Colo., page 95) the conversion of a European process alumina plant to the American process is described. The process selected is similar, with only minor deviations, to the previously referred to FR Pat. No. 1,391,596. The productivity thus achieved is 56.3 g Al.sub.2 O.sub.3 per liter of aluminate liquor to be decomposed, with a reaction time between 40 and 50 hours. Other processes are also referred to in this publication, which, although definitely giving a coarse product, exhibit lower productivity than the described process employed.
Summarizing, previously known proposals for the improvement of the productivity of the American process do not yield much more than approximately 55 grams Al.sub.2 O.sub.3 per liter of the aluminate liquor to be composed.
This value is naturally subject to certain deviations above and below, and is dependent on the initial supersaturation of the aluminate liquor and the time of reaction selected.
When the productivity is compared with that of up to 80 grams Al.sub.2 O.sub.3 per liter from the European process a very considerable difference exists. Accordingly, the purpose of the invention is to improve the decomposition yield (productivity) of aluminum hydroxide in terms of grams of Al.sub.2 O.sub.3 per liter from the clarified, supersaturated sodium aluminate liquor to be decomposed, while obtaining an aluminum hydroxide of coarse particle size (American type) whose fine fraction (less than 45 micrometers) does not exceed 15 percent by weight, and normally exhibits a range of between 4 to 8 percent by weight.